1. Field of the Invention
The invention concerns the field of optical instruments with high, or even very high, resolution.
2. Description of the Prior Art
As the man skilled in the art knows, certain optical instruments have geometrical aberrations that it is increasingly imperative to compensate the higher the required resolution.
At least two solutions have been proposed for compensating these geometrical aberrations.
A first solution consists in using liquid crystals to vary the index of the glass under the action of an electric field. The drawback of this solution resides in the fact that it generates chromatic aberrations, to the extent that it is difficult to apply to multi-spectrum instruments or to instruments with a wide spectral band.
A second solution consists in using deformable (or active) mirrors. Here it is a question of controlling the deformation of the reflecting surface of a mirror at the level of a pupil of the observation instrument whose geometrical aberrations must be compensated.
Thus the University of Delft has proposed placing, at a chosen distance from a substrate including a printed circuit on which are defined electrodes connected to electrical power supply circuits, a suspended reflecting membrane connected to the ground of said electrical power supply circuits. By placing an electrode at a chosen electrical potential, the suspended reflecting membrane is drawn toward that electrode and is therefore deformed.
This solution has at least two drawbacks.
A first drawback is due to the integration of the membrane onto its support. The stresses of fixing the flexible membrane to the circular mount in effect generate deformations that must be minimized mechanically and that may reappear under the effect of temperature variations or local temperature gradients.
A second drawback is due to the electrostatic force applied by the actuators (or electrodes) to the membrane. In effect, this electrostatic force being proportional to the square of the applied voltage (V2), the actuators can exert on the membrane only a traction force that tends to move it toward them. It is consequently necessary to apply an offset voltage to move the plane of the membrane closer to the plane of the electrodes in such a manner as to attempt to create bosses on the membrane by locally relaxing the traction force. This solution imposes using a portion of the actuation capacity of the electrodes. Because of its high curvature, a peripheral ring of the mirror can therefore no longer be used to correct aberrations, which induces a reduction of the usable area. Furthermore, in the event of power supply failure, and therefore disappearance of the offset voltage, the displacement of the reflecting membrane and its curvature variation change the setting of the instrument, which may render the image unusable.
No known solution providing entire satisfaction, an object of the invention is therefore to improve on the situation.